1. Field of the Invention
The present invention relates to inkjet ink compositions comprising modified pigments which are the reaction product of a pigment and a specified reagent, such as a hydrazine reagent, as well as to the modified pigments themselves and methods of preparing them.
2. Description of the Related Art
An inkjet ink composition generally consists of a vehicle, which functions as a carrier, and a colorant such as a dye or pigment. Additives and/or cosolvents can also be incorporated in order to adjust the inkjet ink to attain the desired overall performance properties.
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions which can be used in inkjet printing. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants. Typically, these polymeric dispersants have a molecular weight less than 20,000 in order to maintain solubility and therefore pigment stability.
The surface of pigments contains a variety of different functional groups, and the types of groups present depend on the specific class of pigment. Several methods have been developed for grafting materials and, in particular, polymers to the surface of these pigments. For example, it has been shown that polymers can be attached to carbon blacks containing surface groups such as phenols and carboxyl groups. However, methods which rely on the inherent functionality of a pigment's surface cannot be applied generally because not all pigments have the same specific functional groups.
Modified pigments have also been developed which provide ink compositions with improved properties, such as dispersibility, without the need for an external dispersant. Typically, methods to prepare these modified pigments involve preparing a dispersion of a pigment in a medium and reacting the surface of this pigment with various types of reagents. The resulting product is a modified pigment comprising the pigment having attached at least one organic group, such as an organic group comprising at least one ionic group, ionizable group, or a mixture thereof. For example, modified pigments can be prepared using the methods described in U.S. Pat. Nos. 5,554,739, 5,707,432, 5,837,045, 5,851,280, 5,885,335, 5,895,522, 5,900,029, 5,922,118, and 6,042,643, and PCT Publication WO 99/23174. Such methods provide for a more stable attachment of the groups onto the pigment compared to dispersant type methods, which use, for example, polymers and/or surfactants.
Other methods for preparing modified pigments include reacting a pigment having available functional groups with a reagent comprising the organic group, such as is described in, for example, U.S. Pat. Nos. 6,723,783, 6,911,073, and 7,173,078. Such functional pigments may be prepared using the methods described in the references above. In this way, the pigment is modified to comprise a functional group not inherent to its surface but which can further react with a reagent having the target organic group.
Carbonaceous materials, such as carbon black, have also been shown to react with hydrazine reagents under specific types of conditions. For example, U.S. Pat. Nos. 6,723,783 and 7,173,078, as well as U.S. Patent Application Publication No. 2004/0171725, each relate to the reaction of various types of modified pigments, which comprise a pigment having an attached functional group, with specified reagents, including hydrazine reagents. However, such methods require the initial formation of a modified pigment as a precursor which is then further modified using the hydrazine reagent. Furthermore, it is known that hydrazine can be used as a reducing agent for an oxidized carbon black and graphite oxide, which are carbonaceous materials that have been pretreated with an oxidizing agent to introduce higher levels of oxygen-containing groups. Again, though, this method requires the use of a previously modified carbon black. Also, U.S. Pat. No. 4,835,074 describes the chemical modification of acetylene blacks, with or without added oxygen functionality, with covalently bonded nitrogen-containing groups. These modified acetylene blacks are used for electrochemical cells.
While various types of modified pigments have been prepared, some of which can be used in inkjet ink compositions, there remains a need to provide alternative modified pigments, particularly by reaction of a pigment that is itself not a previously modified pigment.